Removing unused refcount variable
Removing unused refcount variable

The rebranch branch is failing to build due to an unused refcount
variable in runtime.c. The variable is only used by an os_assert and
nowhere else. I've removed it and instead put the check directly in the
assert.

apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch,apple/swift-corelibs-libdispatch